As electronic apparatuses are miniaturized and their performances are increased, it is desirable that mounting techniques be improved in terms of the miniaturization and performance increase.
In circuit substrates used in electronic apparatuses and information processing apparatuses, such as personal computers and servers, logical elements, such as CPUs, are provided with a decoupling capacitor around them so that stable current supply can be ensured and/or so that noise can be removed even if the power-supply voltage fluctuates. This is increasingly important with the increase in operating frequency of CPUs and with the decrease in operating voltage.
In particular, in order to quickly compensate for the fluctuation of the voltage applied to the CPU, it is effective to control the impedance of the power-supply system including the decoupling capacitor. Accordingly, a high capacitance capacitor having a low inductance is desirable, and it is also desirable to reduce the length and hence the inductance of the power-supply wiring to the decoupling capacitor on the circuit substrate.
In order to reduce the inductance of the power supply wiring to the decoupling capacitor on the circuit substrate, it is believed that the most effective approach is to dispose a decoupling capacitor immediately under the CPU and to dispose the decoupling capacitor within the circuit substrate. This structure can lead to a cost reduction in manufacturing semiconductor devices and circuit substrates.
Related art references include the following documents:
Japanese Laid-open Patent Publication No. 2009-524259 (corresponds to US Publication No. 2007/0222030);
Japanese Laid-open Patent Publication No. 05-347227;
Japanese Laid-open Patent Publication No. 2007-318089 (corresponds to US Publication No. 2007/0263364); and
Takeuchi, T. et al., J. Mater. Res. Vol. 18, No. 8, August 2003, pp. 1809-1815.
In a method proposed for incorporating a decoupling capacitor into a circuit substrate, a capacitor component is prepared by firing at a high temperature a multilayer structure including ceramic green sheets on which electrode patterns have been printed, and the resulting capacitor component is embedded in a circuit substrate. Another method has also been proposed in which a capacitor dielectric film is formed for each of a plurality of build-up layers, and the thus formed capacitor dielectric films are stacked in a process for forming a build-up circuit substrate.
Recently, a capacitor has been proposed which includes a platinum lower electrode on a silicon substrate coated with a silicon oxide film, a highly dielectric or ferroelectric ceramic dielectric film formed on the lower electrode by sputtering, a platinum upper electrode on the ceramic dielectric film, and connection electrodes extending upward respectively from the lower electrode and the upper electrode.
For a circuit substrate containing a completed capacitor component, the capacitor component is generally prepared by firing a stack of a plurality of green sheets containing a large amount of organic binder on each of which an electrode pattern has been printed. However, in this instance, the green sheets are significantly shrunk by the firing. Accordingly, it is difficult to form a fine electrode pattern on such a green sheet. For mounting a capacitor component on a circuit substrate, a high-resistance solder is used for bonding. The high-resistance solder increases the impedance of the power-supply system in the entire circuit substrate.
For preparing a capacitor component by a green sheet method, the firing operation for forming a ceramic capacitor dielectric film from a green sheet is performed in an oxidizing atmosphere at a high temperature. Accordingly, capacitor electrodes and via-plugs of the capacitor component are formed of a heat-resistant metal, such as nickel. However, heat resistant metals have higher resistivity than copper used for LSI wiring or the like, and result in increased impedance.
In a capacitor element including a platinum capacitor electrode, the impedance of the element is increased because of the high resistivity of platinum. Also, ceramic dielectric films formed by sputtering, which are amorphous, are heat-treated to be crystallized. However, this heat treatment is likely to cause the dielectric film to crack, and consequently, leakage current can occur undesirably.
From the viewpoint of reducing the impedance, it is desirable that the electrode pattern and via-plugs in the capacitor component be formed of copper, which has low resistivity. However, copper has the melting point of 1084° C. while the firing of known green sheets is performed at a temperature of at least about 1500° C. It is therefore impossible to form an electrode pattern or via-plugs in a capacitor component having a structure in which ceramic dielectric films are disposed on top of one another by a known technique.
In the structure in which capacitor dielectric films are disposed on t of one another in a build-up substrate, the thickness of the entire capacitor becomes the same as the total thickness of the build-up layers. This structure requires longer via-connection and results in an increase in impedance. In addition, this structure increases the number of insulating layers of the circuit substrate, and accordingly increases the number of process steps. Consequently, the manufacturing cost is increased, and, further, the total thickness of the circuit substrate is increased to increase the impedance of the signal line. The capacitor dielectric films for respective build-up layers are formed by a low-temperature process such as sputtering so as to reduce and/or prevent damage to the build-up layers. However, the capacitor dielectric films formed at a low temperature are typically amorphous. Thus, even though a high dielectric material or a ferroelectric material that can originally achieve a high relative dielectric constant of 1000 or more is used, its relative dielectric constant is not more than about 40, and a satisfactory capacitor component cannot be achieved. A build-up substrate may be made of a composite material containing a resin and a high-dielectric ceramic. In this instance, however, the dielectric constant of the composite material is not more than about 50 due to the effect of the resin (typically, epoxy resin) having a low dielectric constant.